Field of the Invention
The present invention relates to an image forming apparatus and more specifically to an image forming apparatus arranged to form an image on an envelope.
Description of the Related Art
Hitherto, an electro-photographic image forming apparatus such as a printer, a facsimile, a copier, and a multi-function printer having functions of the apparatuses described above is configured to form an electrostatic latent image on an image bearing member and to develop the latent image by using toner to form a toner image at first in forming an image on a sheet. Then, the toner image is transferred onto the sheet by means of electrostatic force and the transferred toner image is fixed to the sheet by applying heat and pressure to the toner image on the sheet. The sheet on which the toner image has been fixed is discharged out of the apparatus by a sheet discharge roller. It is noted that in a case in which images are to be formed on both surfaces of the sheet, the sheet on which the image has been formed on a first surface thereof is fed again by a reverse mechanism to the image forming portion through a duplex conveying path. Then, after transferring a toner image on a second surface of the sheet, the toner image is fixed by a fixing unit and then the sheet on which the images have been formed on the both surfaces thereof is discharged out of the apparatus.
By the way, there is a case when the sheet that has passed through the fixing unit causes curl by receiving heat. The curl grows largely in particular at a downstream edge in a sheet conveying direction at both widthwise edge corners orthogonal to the sheet conveying direction. Then, if the curl occurs, there is a possibility that the sheet is caught by a sheet conveyance guide and ribs provided on the sheet conveyance guide, causing corner folding and jamming.
Then, in order to smoothly convey a sheet whose curl on both corners is large, there is proposed an arrangement in which a plurality of ribs provided on a sheet conveyance guide to smoothly convey such sheet is disposed aslant into a fan form, a closure side of the fan form is positioned upstream in an approach direction, and a slope is provided on a rib side surface as disclosed in Japanese Patent Application Laid-open No. 2001-322735.
By the way, there is a case of forming an image on an envelope including an enclosing port and a flap closing the enclosing port as one example of the sheet by the conventional image forming apparatus. Then, there is also a case when the flap is folded such that the flap is located at one widthwise side of the envelope in conveying the envelope to an image forming portion.
In this case, the folded flap is not completely flattened and is kept opened more or less. Due to that, even if the plurality of ribs are disposed aslant into the fan form and the slope is provided on the side surface of the rib, there is a case when the flap is caught and thus causes corner folding, depending on an angle of the slope.